Phase field simulation of columnar grain formation induced by pore migration in UO2

Abstract

This study investigates the formation of radially aligned columnar grains surrounding central voids in porous uranium dioxide (UO₂) fuel rods, utilizing a multiphase field model to analyze the impact of pore migration. The results indicate that the transformation from equiaxed grains to columnar structures is driven by the directional migration of pores towards the fuel center, influenced by high temperatures and steep temperature gradients. Increased temperature gradients, porosity, and pore density enhance the columnar morphological features of the grains. As pores migrate, they attach to grain boundaries, facilitating the movement of circumferential grain boundaries while pinning radial ones. This interaction promotes directional grain coarsening towards the fuel center, ultimately resulting in distinct columnar grains. This research provides valuable insights into the complex interactions between pore dynamics and grain boundary behavior, elucidating the microstructural evolution of UO₂ under operational conditions.